Voluntary striated skeletal muscles are able to achieve a considerable increase in mass during normal postnatal growth and in response to altered functional demands. In the differentiated, developing muscle the increase in mass is due largely to the cross-sectional growth of individual muscle fibers. The total number of muscle fibers does not normally increase once differentiation is complete. However, the duration of postnatal differentiation varies among the different vertebrate classes. As the fibers grow they receive additional nuclei from fusing myogenic cells, synthesize contractile proteins which are then assembled into filaments and incorporated into the myofibril. Unlike the fibers they comprise, myofibrils are able to grow, divide and increase in number. Each myofibril is surrounded by an organized system of internal and external membranes termed the sarcotubular complex. This research project will investigate the role of functional and developmental parameters (amount of fiber shortening and rate of sarco-tubular growth) effecting myofibril growth, division and differentiation in avian skeletal muscle. Electron microscopy will be used to study sarcotubular system growth, fiber size, myofibril size, myofibril number and myonuclei number in the fast and slow muscles of the chicken. Since both muscles are homogenous with respect to fiber type, they are particularly well suited for this investigation. Of particular importance is to what extent continuous changes in contractile filament overlap, i.e., sarcomere shortening influence : l) myofilament synthesis and assembly, 2) myofibril division and 3) sarco-tubular proliferation in developing muscle fiber. Also, the extent to which variation in early contractile activity limits or influences the ability of the mature phasic and tonic myofibers to later adapt to new functional situations.